arch-arm: Adjust breakpoint EC depending on source state

arch-arm: Adjust breakpoint EC depending on source state

The software breakpoint exception class needs to be adjusted depending
on the source EL's execution state. This change fixes an incorrect
exception class when taking a breakpoint from aarch64.

Change-Id: I99d87a04be6bf9ce3a69f6b19969fa006cfd63a4
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Ciro Santilli <ciro.santilli@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/10809
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>